1. Field of the Invention
The present invention relates to an optical pick-up device for optically writing information into and reading out the same from a recording medium layered on an optical disc by projecting a light beam onto the recording medium. More particularly, the invention relates to an objective lens driver, used for an optical pick-up device, in which a movable portion having an objective lens is supported by means of suspension wires.
2. Discussion of Background Art
Generally, an optical pick-up device is composed of an objective lens driver having an objective lens and an optical system for transmitting light to and receiving the same from the objective lens. The optical pick-up device is mounted on a mounting table of an optical system block.
A general objective lens driver, as shown in FIG. 15, includes a movable portion 1, a fixed portion 2, and four elastic supporting members 3. The movable portion 1 includes an objective lens 11, a focus coil 12 and a tracking coil 13. The fixed portion 2 includes a magnetic circuit (magnet and others) 21. The elastic supporting members 3 are fastened at both ends thereof to the movable portion 1 and the fixed portion 2, and support the movable portion 1 in a cantilever fashion. The elastic supporting members 3 being metal suspension wires are disposed such that pairs of elastic supporting members are respectively provided on both sides of the movable portion 1 with respect to the objective lens 11. One ends of the pairs of the elastic supporting members 3 are soldered to holder plates 15, while the other ends of them are soldered to a base plate 23. The holder plates 15 are provided on the right and left sides of a lens holder 14 holding the objective lens 11. The fixed portion 2 is disposed so that the elastic supporting members 3 are parallel to a tangential direction of the disc.
The movable portion 1 may be shifted in a focus direction (perpendicular to the disc surface) when current is fed to the focus coil 12, and in a tracking direction (radial direction of the disc) when current is fed to the tracking coil 13. A measure to damp vibrations of the movable portion 1 is taken. As well illustrated in FIG. 16, damper cases 24 are provided on the front side of the base plate 23 to which the other ends of the elastic supporting members 3 are fastened. The other ends of the elastic supporting members 3, as shown, are passed through the damper cases 24 and the base plate 23, and soldered to the outer side of the base plate 23. The damper cases 24 are filled with gel-like damping material 25. In this case, the elastic supporting members 3 placed in part are stuck with the damping material 25. When the movable portion vibrates, the elastic supporting members 3 move through the damping material within the damper cases. At this time, viscous flow of the damping material acts on the moving elastic supporting members, and the supporting members are deformed. The deformation of the supporting members and the viscous flow of the damping material are utilized for the damping of the vibrations of the movable portion. (This damping technique is disclosed in JP-A-2-232824.)
To secure an exact information writing/reading to and from the optical disc, it is required that the optical axis of the objective lens is perpendicular to the surface of the disc. If the optical axis of the objective lens is tilted with respect to the disc surface during a movement of the movable portion (including the objective lens) of the objective lens driver in the focus direction, coma occurs in the optical system and consequently a signal jitter increases. A tangential directional component and a radial directional component make up the tilt of the objective lens. To secure an exactness of the information writing/reading, tilts of those directional components need to be eliminated.
For this reason, in the objective lens driver, the objective lens is mounted on the mounting table such that the optical axis of the objective lens is perpendicular to the disc surface. To this end, the supporting mechanism of the movable portion is designed such that the angular relation of the objective lens of the disc surface is maintained irrespective of the moving directions of the movable portion, the focus direction and the tracking direction.
In the objective lens driver, referred to above, in which the movable portion is supported by the elastic supporting members, the perpendicularity of the optical axis of the objective lens to the disc surface is maintained irrespective of the moving direction of the movable portion if the elastic supporting members have equal lengths and the spatial intervals between both ends of the elastic supporting members are equal.
To prevent the movable portion 1, or the objective lens 11, from being tilted when the movable portion is moved in the focus direction or the radial direction, the background art mentioned above has the following construction: the distances between the fixing ends of the elastic supporting members 3 in the movable portion 1 and the fixing ends thereof in the fixed portion 2 are selected to be equal and those elastic supporting members 3 are disposed to be parallel to one another in the vertical and horizontal directions. Further, the amounts of the damping material 25 contained in the damper cases 24 are selected to be equal to each other on the assumption that the elastic supporting members 3 are fixed at predetermined positions. Spring constants of the elastic supporting members 3 are selected to be equal to one another. When the movable portion 1 is moved in the focus direction, it can be considered that a focus-directional drive force acts on the center of gravity of the movable portion 1. Hence, the gravity center position is coincident with the focus-directional drive center position.
To suppress the resonance in a low frequency region, damping material is put around each wires in the objective lens driver. Use of only the damping material fails to satisfactorily suppress the resonance in a high frequency region by pitching or yawing, however. To cope with this, JP-A-7-105551 and JP-A-9-190636 disclose objective lens drivers in that with the intention of improvement of the high-frequency resonance suppression, the movable portion 1 is supported with the fixed portion 2 in a state that the elastic supporting members 3 are bent in advance in radial direction, as shown in FIG. 17.
In the structure where the elastic supporting members 3 are arcuately bent in advance, the movable portion 1 unavoidably tilts when the movable portion 1 is shifted in the focus direction, even if the spatial intervals between the fixing points of the four elastic supporting members 3 are set to be equal to one another, and those members are disposed strictly parallel to each other. In case where the elastic supporting members 3 are bent in the radial direction, for example, when the movable portion 1 is shifted in the focus direction, its tilting in the tangential direction increases. Particularly when the damping resonating with high frequencies is increased by increasing a quantity of the bending of the elastic supporting members, a tilt of the movable portion 1 in the tangential direction increases. When the tilt of the movable portion 1, i.e., the tilt of the objective lens 11, increases, coma is produced and readout signal jitter increases.
Where the quantity of the bending of the elastic supporting members 3 is reduced with the intention of reducing the tilt of the movable portion 1 in the tangential direction when the movable portion 1 is shifted in the focus direction, the damping effect for the high frequency resonance is lowered. This is problematic when it is assembled into a system.
Thus, the decrease of the tilt of the movable portion 1 in the tangential direction contradicts the increase of the damping for the high frequency resonance suppression.
In the above-mentioned objective lens driver, when the mounting positions of the four elastic supporting members 3 are displaced from the correct ones, a problem arises. The problem arises even if one mounting position is displaced from the correct one. For example, when the space or distance between the upper and lower elastic supporting members 3 on the radial (+) side is different from that on the radial (-) side, a dynamic balance of the structure with respect to the objective lens is lost. When the movable portion 1, which is horizontal at the neutral position as shown in FIG. 19, is shifted in the focus direction, a moment is generated about the gravity center of the movable portion 1, and as shown in FIG. 20, the movable portion 1 is tilted in the radial direction. Under this condition, when the movable portion 1 is shifted in the focus direction, coma is produced and the jitter of a readout signal increases.
For this reason, to prevent the tilt of the movable portion in the radial direction, it is required that the elastic supporting members 3 are highly accurately positioned. It is very difficult to highly accurately position the elastic supporting members 3 in the manufacturing stage. Actually, the resultant products inevitably suffer from the tilt of the movable portions. A possible measure to correct this is to do over again the soldering of the elastic supporting members 3 already fastened by soldering. However, the measure is accompanied by the following disadvantages: production yield is degraded, perfect correction is not always achieved, and product reliability will be impaired at the soldering portions of the elastic supporting members 3.
Next, details of the shift of the movable portion 1 in the focus direction will be given. When a focus-directional drive force F causes the movable portion 1 to shift in the focus (+) direction (toward the disc) as shown in FIG. 21A, a force to cause the elastic supporting members 3L and 3R to return to their original positions acts on those members. Let spring constants of the elastic supporting members 3L and 3R be Kl and Kr. When the movable portion is shifted from the neutral position in the focus direction by a distance X, a force Fl (=-Kl*X) acts on the fixing terminal of the elastic supporting member 3L and a force Fr (=-Kr*X) acts on the fixing terminal of the elastic supporting member 3R. Rotational moments generated about the gravity center G of the movable portion 1, caused by those forces, is expressed by EQU Ml=Fl+L, and Mr=Fr*L
where L=distance between the gravity center G and the fixing terminal of each of the elastic supporting members 3L and 3R.
Those moments Ml and Mr are opposite to each other with respect to the gravity center G. As recalled, the spring constants of the rotational moments Ml and Mr are equal to each other. Hence, the rotational moment Ml that is caused about the gravity center by the force applied from the elastic supporting member 3R is equal to the rotational moment Mr caused by the elastic supporting member 3L. Therefore, the movable portion 1 is not rotated.
Then, let us consider a case that the movable portion 1 is shifted in the radial direction, specifically, it is shifted a distance "l" to the left. In this case, the gravity center G of the movable portion 1 is shifted a distance equal to the radial shift of 21" with respect to the focus-directional drive center position. As a result of the shift, the spring constants Kl and Kr of the elastic supporting members 3L and 3R remain unchanged, viz., those are equal to each other.
The focus-directional drive center position can be considered to be the center of the magnetic circuit 21 of the fixed portion 2. Then, if the movable portion is shifted in the focus direction after it is shifted the distance of the radial shift of "l", a shift of "l" is produced between the focus-directional drive center position on which the focus-directional drive force F acts and the gravity center G of the movable portion 1. As a consequence, a rotation moment Mf (=F*l) is generated about the gravity center G in the movable portion 1. The direction of the rotation moment Mf is a counterclockwise direction when the movable portion is shifted in the focus (+) direction (toward the disc) since the gravity center G is located on the left-hand side when it is viewed from the focus-directional drive center position on which the focus-directional drive force F acts. Therefore, the movable portion 1 is rotated in the counterclockwise direction. In contrast with the above case, when the movable portion is shifted in the focus (-) direction (apart from the disc), the direction of the rotation moment Mf is a clockwise direction. The movable portion 1 is turned in the clockwise direction.
This results in a radial-directional tilt of the movable portion 1. There is known a technique that to remove the tilt, the rotational moment is balanced when the focus-directional drive center position is radially shifted in a manner that a distribution of magnetic flux of the magnetic circuit is shaped like a twin-mountain configuration by dividing the magnetic circuit in the radial direction (disclosed in JP-A-8-50727, for example).
A complicated magnetic circuit is required for varying the focus-directional drive center position in accordance with a quantity of its shift. Use of the complicated magnetic circuit cannot reduce the rotational moment sufficiently, however. Therefore, the movable portion 1 is tilted in the radial direction at the time of the radial shift of the focus-directional drive center position. This results in coma in the optical system, and increase of the jitter of the readout signal.